The invention relates to the field of devices that process products by freeze-drying. More particularly, the invention relates to devices that perform bulk freeze-drying. The invention also relates to a method of bulk freeze-drying.
The invention has a particularly advantageous application in the fields of pharmaceutical preparation and food preparation, and more generally for all high value-added industries that need a preservation method by freeze-drying. For example, the invention can be implemented in the field of biotechnology for the production of inoculum with a view to fermentation of the biomass, in the area of foodstuffs for the freeze-drying of fruits, vegetables, beverages and food preparations, in the area of health for freeze-drying of proteins, peptides, enzymes, bacteria, viruses, living cells, sensitive formulations of antibodies or sensitive molecules, plasma fraction or formulation of sensitive polymers.
Freeze-drying is a low temperature dehydration operation that consists of eliminating by sublimation most of the water contained in a product. Freeze-drying makes it possible to obtain high quality end products without degrading the structure, while preserving a large part of the activity of the microorganisms or cells. Freeze-dried products are able to be preserved for a long time due to the decreased activity of the water in the product.
Indeed, by decreasing the activity of the water in the product, no living organism can grow and all of the chemical reactions that take place in water cannot occur. The very low activity of the water also makes it possible to block any microbiological growth activity. Thus, the form and aspect of freeze-dried products are well preserved and their aromatic qualities are far superior to those of products dried by methods of atomization, fluidized bed or simple drying by evaporators with several effects.
Moreover, the transition of products from the frozen state to the dehydrated state, in the absence of a high proportion of liquid water, reduces the possibilities of development of alteration reactions. Another major technological advantage of freeze-drying is found in the capacity of freeze-dried products to be rehydrated instantaneously thanks to the microscopic pores formed by the vapor during sublimation of the water.
However, the use of freeze-drying is limited by its cost and remains far less used than drying. The low productivity of freeze-drying is due to the discontinuous mode of operation under a vacuum and at very low temperature, which results in significant processing times of between 10 hours and several days. Under these extreme conditions, heat transfers have a very low efficiency. Comparatively, drying is conventionally performed at atmospheric pressure at moderate temperatures, generally between 50 and 100° C., and a heat transfer with better efficiency. Thus, the investment and operating expenses of freeze-drying devices are high. For example, the energy consumption of a freeze-drying device is typically on the order of 2500 to 6000 kWh per ton of water to eliminate.
Consequently, freeze-drying is only applied for products having a high added value. In food industries, coffee can be mentioned, as well as herbs and spices, cooked dishes, or ingredients sensitive to dehydration from heat (vegetables, fruits, seafoods, etc.). Drying methods based on atomization or fluidized bed are currently used for instant dehydrated soups, culinary preparations and breakfast cereals because they are much less expensive. The pharmaceutical industries (vaccines, serum, drugs) and bio-industries (leavens) have a much greater interest in the freeze-drying method, which alone enables them to obtain the most characteristic property of the technique, namely the preservation of an active ingredient (biological and/or drug activity) in a product that will be stored at a temperature close to ambient temperature.
Freeze-drying requires the use of a device composed of a freezing chamber connected to cooling means, an evaporation chamber connected to heating means and a condensing chamber connected to the evaporation chamber. The condensing chamber is configured to collect the water vapor produced from the evaporation chamber onto an ice trap. In the field of pharmacy, for reasons of sterility, the evaporation chamber also freezes the products prior to evaporation. On the contrary, in the field of food, the freezing is conventionally performed in an independent apparatus, so that the freeze-drying device itself only includes an evaporation chamber as well as a condensing chamber.
Cooling means are provided in the condensing chamber to freeze the water vapor coming from the evaporation chamber. The water in vapor form is then transformed into ice in the condensing chamber and the ice is stored in the condensing chamber on the ice trap. In some cases, the freezing and sublimation can be performed within the same enclosure. In this scenario, the freezing chamber and the evaporation chamber consist of a single chamber connected to the cooling means and to the heating means. Preferably, the chambers are also placed under vacuum by a vacuum pump so as to pass below the triple point of water and enable the water to change over from the solid phase to the gas phase.
The freeze-drying method has a first step consisting of freezing the products in the freezing and evaporation chamber to enable their drying at low temperature. Rapid freezing is desired so as to form small ice crystals. Freezing that is too slow results in favoring the formation of large crystals likely to damage the structure of the product by tearing the walls of its cells, for example for yeasts, viruses, and animal or vegetable cells. A second step consists of creating a vacuum in the evaporation chamber, the low pressure, generally well below 6.1 hPa, so the water in the form of ice can transform into vapor without thawing the products. The products receive a supply of heat to furnish the energy needed for the latent heat for sublimation of the ice into vapor. The vapor enters the condensing chamber, which is conditioned to transform the water vapor into ice through use of an ice trap maintained at very low temperature, generally −60° C.
This freeze-drying method therefore makes it possible to extract up to 95% of the water contained in the products. Freeze-drying can make it possible to lower the moisture content of the product to an extremely low level, between 1% and 10% of the volumetric weight of the product, and to prevent bacteria and mold from proliferating and enzymes from triggering chemical reactions likely to breakdown the product. Thus, freeze-dried products are preserved for a very long time. When hermetically packed, protected from humidity, light and oxygen, freeze-dried products can be preserved at ambient temperature for many years. Furthermore, high quality sterilized products also require sterilization of the sterilization chain.
However, the freeze-drying process has a number of disadvantages related to the required major inputs of heat and cooling, to placing the evaporation and condensing chambers under a vacuum, and the need to ensure sterilization of these chambers. The necessary inputs of heat and cooling require the use of highly efficient elements, functioning for example with liquid nitrogen. Placing the chambers under a vacuum and the needs for sterilization require the use of a sealed enclosure and a vacuum pump. Moreover, during sublimation there is a risk of clumping of products that deteriorates the quality of the freeze-dried products.
In addition, the freeze-drying time depends on the size of the particles of the products to be freeze-dried and on the surface area of the products coming into contact with the heat source. A conventional solution consists of distributing the products to be freeze-dried into small vials. The heat source is configured to heat the base of the vials so as to transmit the heat to all of the products stored in the vials by conduction and radiation. After freeze-drying, the product appears in the form of a porous cake in the shape of the vial. The average time for freeze-drying is therefore between two and three days due to the migration time of the heat by conduction and radiation in the vials. However, the distribution of the products to be freeze-dried in a large number of vials requires an evaporation chamber of very large size. Consequently, the power of the heating means, cooling means and vacuum creating means must be increased.
International patent application No. 2012/018320 proposes to reduce the freeze-drying time by implementing bulk freeze-drying by increasing the contact surface between the products and the heat source. More specifically, this patent application discloses a cyclone chamber having a propeller configured to drive the products in a cyclonic motion during freeze-drying. Although this device makes it possible to freeze dry products in bulk, it is particularly complex to implement under a vacuum.
With bulk freeze-drying, an average freeze-drying time of between 5 and 50 hours can be achieved. The reduction of freeze-drying time makes it possible to reduce consumption, production time and therefore production costs. Moreover, limiting the freeze-drying time reduces the exposure of the product to heat. This allows the quality of the freeze-dried product to be improved.
The documents WO 82/02246 and EP 1,236,962 describe freeze-drying chambers the evaporation chamber whereof is rotatable. However, these devices require a complete stoppage of the evaporation chamber in order to add and extract the products. Indeed, the evaporation chamber in these documents is placed under a vacuum during the freeze-drying, and adding and extracting products requires the return to atmospheric pressure and the opening of a sealed wall. Thus, the means for adding and extracting products are particularly long and complex.
The documents EP 2,578,975 and EP 2,578,976 also propose reducing the freeze-drying time by implementing a bulk freeze-drying. To do this, the evaporation chamber is mounted on an axle rotated during freeze-drying. The evaporation chamber is mounted in a sterile enclosure and the axle of the chamber extends from the enclosure through an opening in order to be driven by a motor. A seal is placed around the axle at the opening of the enclosure in order to guarantee the vacuum of the enclosure without loss of pressure at the opening. The seal is configured to withstand pressures of 2.5 bars at temperatures varying between −60 and 120° C.
To carry out the freeze-drying, an operator connects a sterile inlet to the evaporation chamber, passing through the sterile enclosure so as to reach the receptacle. The products to be freeze-dried are then disposed in the receptacle by passing through the sterile inlet and the sterile enclosure. The operator then disconnects the inlet, taking care to preserve the sterility in the enclosure. Freeze-drying is then carried out while the motor rotates the receptacle so as to agitate the products to prevent clumping thereof. The evaporation and condensing chambers are in communication but do not turn. When the freeze-drying is completed, the operator connects a sterile outlet to the evaporation chamber by the sterile enclosure so as to extract the freeze-dried products from the receptacle.
Due to the pressures utilized and the difference of temperatures, the seal around the axle quickly degrades, which can cause a loss of seal or sterility. Furthermore, this freeze-drying device also requires very precise handling by the operator in order to ensure the sterility of the products.
Moreover, the freeze-drying devices require steps of handling by an operator between two freeze-dryings. The result is that the freeze-drying is a process that is mostly not automated, thus increasing production time and therefore the cost of freeze dried products.
The problem of the invention therefore consists of developing a device for freeze-drying products in bulk that responds to the disadvantages of the devices of the prior art.